There is disclosed a continuously variable transmission including a hydraulic circuit that includes a first oil pump configured to pump up oil from an oil pan to generate a line pressure and a second oil pump configured to be provided between a primary pulley and a secondary pulley to adjust the oil in/out to/from a primary oil chamber.
Japanese Publication of a Translation of an International Application 2002-523711 discloses that a shifting oil passage branched from a secondary oil passage for supplying line-pressure oil to a secondary pulley is provided and a second oil pump is provided in the shifting oil passage. The hydraulic circuit controls a speed ratio by adjusting the amounts of oil to be supplied to the primary pulley and the secondary pulley because the oil in/out to/from the primary oil chamber is adjusted by controlling the rotation direction of the second oil pump.
In such the continuously variable transmission, the second oil pump is driven by an electric motor. However, when a contamination etc. is mixed in oil, there is a possibility that the contamination is caught in a movable part of the oil pump and the rotation of the oil pump cannot be accurately controlled. When the rotation of the second oil pump cannot be controlled, the oil in/out to/from the primary pulley cannot be appropriately performed and a desired speed ratio cannot be realized. The catch of a contamination into the movable part of the oil pump becomes easy to occur along with the miniaturization of the recent oil pump.
An object of the present invention is to provide a method for controlling a continuously variable transmission and a continuously variable transmission system in which the possibility that the rotation of an oil pump cannot be accurately controlled is reduced.